1. FIELD OF THE INVENTION
This invention relates to the protection of aluminum and aluminum alloys by the deposition of a polymeric coating onto the aluminum surface which is chemically bonded to the aluminum through aluminum oxide on its surface.
2. DESCRIPTION OF THE PRIOR ART
Aluminum is an excellent structural material because of its low cost and great strength per unit weight. Aluminum has chemical characteristics, however, which make it subject to corrosion, particularly by salt water and/or salt spray. The corrosion takes the form of a white rust (aluminum oxide) and the aluminum finish itself is easily spoiled by scratching or abrasion due to its inherent softness. Further, the painting of aluminum generally does not provide satisfactory corrosion resistance since the aluminum oxide layer under the paint prevents good bonding of the paint to the aluminum surface. Accordingly, painting is generally not a satisfactory corrosion inhibitor for aluminum.
The principal prior art method of protecting the finish of aluminun and aluminum alloys is that of anodizing. The process generally is performed by the immersion of aluminum in a sulfuric or chromic acid bath with the aluminum piece being the anode in an electrolytic process. This anodization forms a hard aluminum oxide coating on the aluminum surface, but the coating itself is porous and undesirably absorptive. Accordingly, the aluminum oxide layer formed by anodization may be further sealed by hydration in hot water. Another prior art approach is the use of sodium dichromate as a corrosion inhibitor and seal. The sodium dichromate provides improved corrosion resistance but leaves the coating a greenishyellow color. The corrosion inhibiting chromate ions are absorbed in the aluminum oxide matrix and are sealed in place by the formation of the hydrate. The greenish-yellow color is undesirable for many applications.
The prior art approaches thus have one or more drawbacks of unwanted color or lack of sufficient abrasion and corrosion resistance or stain resistance. Further, these conventional coating processes are usually inadequate because the bonding is physical in nature and the coating can become mechanically dislodged. The porosity of prior art coatings also is a major problem in preventing corrosion over a long period of time.
Accordingly, it is an object of the present invention to provide a process for the sealing of aluminum surfaces through the graft polymerization of monomers to the aluminum surface through the aluminum oxide on the aluminum surface.